A tiltrotor aircraft is one which typically has a pair of pods located on its wings for supporting a pair of rotors, the pods being movable between a vertical position when the rotors serve as helicopter rotors for vertical take-offs and landings, and a horizontal position when the rotors serve as propellers for forward flight.
Generally, a large rotor diameter is advantageous for operating the aircraft in the helicopter mode to provide low disk loading which results in efficient operation, low noise levels and diminished downwash velocities.
On the other hand, a relatively small diameter is advantageous in the propeller mode to reduce tip speed and blade area for improved propulsion efficiency, minimized blade aero-elastic properties and to simplify ground handling.
Various mechanisms are known for providing the means for extending and retracting variable diameter rotor blades. For example, in U.S. Pat. No. 3,768,923, a jackscrew and nut arrangement is used to produce telescopic motion between inner and outer blade portions. Referring to FIG. 1, two coaxial shafts A and B are connected to upper and lower bevel gears C and D to form part of a differential unit which includes pinion gears E connected to the jackscrew shafts F in each blade. The coaxial shafts rotate with the rotor drive shaft G during constant diameter operation. However, each shaft has a brake H and I, respectively such that when the brake is applied, the selected shaft and rotor rotate at different speeds. Depending on which shaft undergoes braking, this speed differential drives the respective bevel gear and jackscrews for rotation, with rotation in opposite directions depending on which brake is applied to which coaxial shaft. Thus, application of a brake to one or the other shafts will drive the jackscrew to either extend or retract the rotor blade.
While such an assembly is acceptable in some circumstances, such as with articulated or hingeless rotor hubs, it is difficult to incorporate into a gimballed rotor hub which must tilt relative to the rotor drive shaft, so that both internal coaxial shafts would also be required to have a degree of flexibility or articulation. This is difficult to provide in a limited space, and adds weight and maintenance penalties.